This invention relates to an electron beam deflection control system which is suitable for use in an electron beam exposure system, a graphic display system, and the like.
An electron beam exposure system functions to form a minute pattern of a semiconductor device or circuit on a mask or a semiconductor wafer with an electron beam. In order to draw the necessary pattern with the electron beam, the system employs an electron beam deflection control circuit which deflects the electron beam linearly at a uniform speed and which, upon completion of the drawing of one pattern, cuts off the electron beam and moves it to the starting point of the next pattern. The electron beam deflection control system of this type has been provided as a digital system and an analog system in the prior art.
In one prior art electron beam deflection control system of the digital type, coordinate data corresponding to respective points of a line pattern are sequentially generated in accordance with data fed from a digital computer, they are converted into an analog signal having a stepped shape by a digital-to-analog converter, and the electron beam is deflected stepwise. With such prior art system, however, the settling time of the digital-to-analog converter which determines each step width of the stepped signal is long. Disadvantageously, therefore, as the line pattern becomes longer, the drawing time becomes longer.
On the other hand, in an electron beam deflection control system of the analog type, a closed loop circuit consisting of a differential amplifier having a limiter function and an integrator is provided, start point and end point levels of a drawing pattern generated by a digital-to-analog converter are sequentially impressed on the closed loop circuit, and the outputs of the integrator are converged to the end point level, thereby to produce a triangular deflection signal. With such an analog system, however, the outputs of the integrator becomes dull in the vicinity of the start and end points of the drawing patern, and therefore, degrade the precision of the drawing pattern. Besides, the convergence time of the first-order lag system formed by the differential amplifier and the integrator increases as a dead time. Therefore, in the case where the drawing pattern is short, the dead time becomes significant as compared with the drawing time. Since the triangular deflection signal is used also for the movement of the beam between the patterns, the dead time increases still more.